Locant

In the nomenclature of organic chemistry, a locant is a term to indicate the position of a functional group or substituent within a molecule.

[1] The International Union of Pure and Applied Chemistry (IUPAC) recommends the use of numeric prefixes to indicate the position of substituents, generally by identifying the parent hydrocarbon chain and assigning the carbon atoms based on their substituents in order of precedence.

For example, there are at least two isomers of the linear form of pentanone, a ketone that contains a chain of exactly five carbon atoms.

Any side chains can be present in the place of oxygen and it can be defined as simply the number on the carbon to which any thing other than a hydrogen is attached.

The α-carbon (alpha-carbon) refers to the first carbon atom that attaches to a functional group, such as a carbonyl.

The second carbon atom is called the β-carbon (beta-carbon), the third is the γ-carbon (gamma-carbon), and the naming system continues in alphabetical order.

These groups give the α-carbon its stereogenic properties for every amino acid except for glycine.